RTLD-AUDIT

NAME

SYNOPSIS

DESCRIPTION

The GNU dynamic linker (run-time linker)
provides an auditing API that allows an application
to be notified when various dynamic linking events occur.
This API is very similar to the auditing interface provided by the
Solaris run-time linker.
The necessary constants and prototypes are defined by including
<link.h>.

To use this interface, the programmer creates a shared library
that implements a standard set of function names.
Not all of the functions need to be implemented: in most cases,
if the programmer is not interested in a particular class of auditing event,
then no implementation needs to be provided for the corresponding
auditing function.

To employ the auditing interface, the environment variable
LD_AUDIT
must be defined to contain a colon-separated list of shared libraries,
each of which can implement (parts of) the auditing API.
When an auditable event occurs,
the corresponding function is invoked in each library,
in the order that the libraries are listed.

la_version()

unsigned int la_version(unsigned int version);

This is the only function that
must
be defined by an auditing library:
it performs the initial handshake between the dynamic linker and
the auditing library.
When invoking this function, the dynamic linker passes, in
version,
the highest version of the auditing interface that the linker supports.
If necessary, the auditing library can check that this version
is sufficient for its requirements.

As its function result,
this function should return the version of the auditing interface
that this auditing library expects to use (returning
version
is acceptable).
If the returned value is 0,
or a version that is greater than that supported by the dynamic linker,
then the audit library is ignored.

la_objsearch()

The dynamic linker invokes this function to inform the auditing library
that it is about to search for a shared object.
The
name
argument is the filename or pathname that is to be searched for.
cookie
identifies the shared object that initiated the search.
flag
is set to one of the following values:

LA_SER_ORIG

This is the original name that is being searched for.
Typically, this name comes from an ELF
DT_NEEDED
entry, or is the
filename
argument given to
dlopen(3).

LA_SER_LIBPATH

name
was created using a directory specified in
LD_LIBRARY_PATH.

LA_SER_RUNPATH

name
was created using a directory specified in an ELF
DT_RPATH
or
DT_RUNPATH
list.

As its function result,
la_objsearch()
returns the pathname that the dynamic linker should use
for further processing.
If NULL is returned, then this pathname is ignored for further processing.
If this audit library simply intends to monitor search paths, then
name
should be returned.

la_activity()

void la_activity( uintptr_t *cookie, unsigned int flag);

The dynamic linker calls this function to inform the auditing library
that link-map activity is occurring.
cookie
identifies the object at the head of the link map.
When the dynamic linker invokes this function,
flag
is set to one of the following values:

LA_ACT_ADD

New objects are being added to the link map.

LA_ACT_DELETE

Objects are being removed from the link map.

LA_ACT_CONSISTENT

Link-map activity has been completed: the map is once again consistent.

la_objopen()

The dynamic linker calls this function when a new shared object is loaded.
The
map
argument is a pointer to a link-map structure that describes the object.
The
lmid
field has one of the following values

cookie
is a pointer to an identifier for this object.
The identifier is provided to later calls to functions
in the auditing library in order to identify this object.
This identifier is initialized to point to object's link map,
but the audit library can change the identifier to some other value
that it may prefer to use to identify the object.

As its return value,
la_objopen()
returns a bit mask created by ORing zero or more of the
following constants,
which allow the auditing library to select the objects to be monitored by
la_symbind*():

LA_FLG_BINDTO

Audit symbol bindings to this object.

LA_FLG_BINDFROM

Audit symbol bindings from this object.

A return value of 0 from
la_objopen()
indicates that no symbol bindings should be audited for this object.

la_objclose()

unsigned int la_objclose(uintptr_t *cookie);

The dynamic linker invokes this function after any finalization
code for the object has been executed,
before the object is unloaded.
The
cookie
argument is the identifier obtained from a previous invocation of
la_objopen().

In the current implementation, the value returned by
la_objclose()
is ignored.

la_preinit()

void la_preinit(uintptr_t *cookie);

The dynamic linker invokes this function after all shared objects
have been loaded, before control is passed to the application
(i.e., before calling
main()).
Note that
main()
may still later dynamically load objects using
dlopen(3).

The dynamic linker invokes one of these functions
when a symbol binding occurs between two shared objects
that have been marked for auditing notification by
la_objopen().
The
la_symbind32()
function is employed on 32-bit platforms;
the
la_symbind64()
function is employed on 64-bit platforms.

The
sym
argument is a pointer to a structure
that provides information about the symbol being bound.
The structure definition is shown in
<elf.h>.
Among the fields of this structure,
st_value
indicates the address to which the symbol is bound.

The
ndx
argument gives the index of the symbol in the symbol table
of the bound shared object.

The
refcook
argument identifies the shared object that is making the symbol reference;
this is the same identifier that is provided to the
la_objopen()
function that returned
LA_FLG_BINDFROM.
The
defcook
argument identifies the shared object that defines the referenced symbol;
this is the same identifier that is provided to the
la_objopen()
function that returned
LA_FLG_BINDTO.

The
symname
argument points a string containing the name of the symbol.

The
flags
argument is a bit mask that both provides information about the symbol
and can be used to modify further auditing of this
PLT (Procedure Linkage Table) entry.
The dynamic linker may supply the following bit values in this argument:

A previous
la_symbind*()
call returned an alternate value for this symbol.

By default, if the auditing library implements
la_pltenter()
and
la_pltexit()
functions (see below), then these functions are invoked, after
la_symbind(),
for PLT entries, each time the symbol is referenced.
The following flags can be ORed into
*flags
to change this default behavior:

LA_SYMB_NOPLTENTER

Don't call
la_pltenter()
for this symbol.

LA_SYMB_NOPLTEXIT

Don't call
la_pltexit()
for this symbol.

The return value of
la_symbind32()
and
la_symbind64()
is the address to which control should be passed after the function returns.
If the auditing library is simply monitoring symbol bindings,
then it should return
sym->st_name.
A different value may be returned if the library wishes to direct control
to an alternate location.

la_pltenter()

The precise name and argument types for this function
depend on the hardware platform.
(The appropriate definition is supplied by
<link.h>.)
Here is the definition for x86-32:

This function is invoked just before a PLT entry is called,
between two shared objects that have been marked for binding notification.

The
sym,
ndx,
refcook,
defcook,
and
symname
are as for
la_symbind*().

The
regs
argument points to a structure (defined in
<link.h>)
containing the values of registers to be used for
the call to this PLT entry.

The
flags
argument points to a bit mask that conveys information about,
and can be used modify subsequent auditing of, this PLT entry, as for
la_symbind*().

The
framesizep
argument points to a
long int
buffer that can be used to explicitly set the frame size
used for the call to this PLT entry.
If different
la_pltenter()
invocations for this symbol return different values,
then the maximum returned value is used.
The
la_pltenter()
function is only called if this buffer is
explicitly set to a suitable value.

The return value of
la_pltenter()
is as for
la_symbind*().

la_pltexit()

The precise name and argument types for this function
depend on the hardware platform.
(The appropriate definition is supplied by
<link.h>.)
Here is the definition for x86-32:

This function is called when a PLT entry,
made between two shared objects that have been marked
for binding notification, returns.
The function is called just before control returns to the caller
of the PLT entry.

The
sym,
ndx,
refcook,
defcook,
and
symname
are as for
la_symbind*().

The
inregs
argument points to a structure (defined in
<link.h>)
containing the values of registers used for the call to this PLT entry.
The
outregs
argument points to a structure (defined in
<link.h>)
containing return values for the call to this PLT entry.
These values can be modified by the caller,
and the changes will be visible to the caller of the PLT entry.

In the current GNU implementation, the return value of
la_pltexit()
is ignored.

CONFORMING TO

This API is non-standard, but very similar to the Solaris API,
described in the Solaris
Linker and Libraries Guide,
in the chapter
Runtime Linker Auditing Interface.

NOTES

Note the following differences from the Solaris dynamic linker
auditing API:

*

The Solaris
la_objfilter()
interface is not supported by the GNU implementation.

*

The Solaris
la_symbind32()
and
la_pltexit()
functions do not provide a
symname
argument.

*

The Solaris
la_pltexit()
function does not provide
inregs
and
outregs
arguments (but does provide a
retval
argument with the function return value).